1. Field of the Invention
This invention relates to an optical apparatus for a line scanner system, and more particularly, to an optical apparatus which provides a relative wider field angle resulting in reduced optical total track and high image quality for a line scanner system.
2. The Related Art
It is well known in the field of optical design that the application of a low power optical element near or at an image plane is effective in correcting residual field aberration. Such concept is commonly applied to designs such as projections, reconnaissance lenses that require high flat field optical performance. Depending on its effect in the correction of residual field curvature as well as astigmatism and distortion, a field lens is also commonly known as a field flattener.
Field flatteners were first used in the Petzval type of lenses. The classic Petzval type lenses are described in U.S. Pat. No. 65,729, German Pat. No. 5761, Great Britain Pat. No. 4756 and U.S. Pat. No. 1,479,251. The classic Petzval type lenses suffer from residual astigmatism and field curvature. With the introduction of a suitably designed field flattener, a much more favorable image performance can be obtained, without having to resort to a significantly more complex optical design. Such systems are described in Great Britain Pat. No.127058, U.S. Pat. Nos. 2,076,190, 2,541,484 and 2,649,021. The field flattener lenses in these systems are of spherical construction in which the profiles of the optical surfaces are spherical. Further, such field flatteners are positioned between the main converging lens group and the focal plane and close to the focal plane, thereby accomplishing the aberration correction requirement.
Without incorporating a field lens, a lens group could be designed to have similar flat field performance. However such lens group would normally have a more complex design. More lens elements would be needed to accomplish the same level of optical aberration correction as in a lens group with a field flattener. The introduction of a field lens in the optical design is an elegant way of simplifying a lens group, which would otherwise be a complex design, while maintaining a high level of optical performance.
An aspherical profile can also be applied to the field flattener with the added benefit of further improvement in aberration correction. The result is a high performance optical system. An example of such system is disclosed in U.S. Pat. No. 2,803,997. In the same patent, a system with relatively wide angle of field of view was disclosed using the field flattener techniques, indicating that the use of aspherical field flattener can be advantageous in aberration correction in wide-angle applications.
The aforementioned examples are optical apparatus designs corrected for a sufficiently broad spectrum, for example, the visible spectrum. There also exists a class of optical apparatus that incorporates field flatteners in the close proximity of the image plane, whereby the optical aberration correction are limited to within a narrow spectrum band. For instance, some lenses each having at least an aspherical profile can be applied to optical apparatuses in monochromatic laser scanning applications, as a field flattener with the added benefit of aberration correction. Some typical examples of such optical apparatus adapted for use in a laser scanning system are described in U.S. Pat. Nos. 5,179,465 and 6,535,317. In these patents, it is characterized that a laser light source is utilized to emit a narrow wavelength band light beam and a rotating polygon mirror is used to reflect and put the light beam into a sweeping motion in the main scanning plane so that the instantaneous field of view is a spot. In addition, a lens in the form of a strip having at least an aspherical surface is arranged for aberration correction as a field flattener. Further, such optical apparatus has to be designed to realize satisfactory f-theta characteristics of distortion correction. Typically, the aspherical profile is bi-laterally symmetrical, following the mathematical description of a toric surface. The overall optical design is not suitable for broadband imaging applications.
Optical total track is defined as the axial distance from the object plane to the image plane. This particular optical property has great significance in a finite conjugate imaging system in which the distance between the object plane and the lens system is of finite value. In addition, the distance between the lens and the image plane generally has a finite fixed value. Therefore the optical total track is generally a fixed value. For first order approximation in optical calculation, given a fixed object size and magnification ratio, also known as reduction ratio, the total track is inversely proportional to the tangent of the field angle. A short optical total track means wide angular field. Generally, for a line-scanning device, such as a flatbed document scanner, it is desirable to have a short optical total track, which will afford a compact packaging of the device. This poses a specific difficulty in the design of the image forming optics. Due to the short total track requirement, the image forming optics would have to have a wide angular field of view while providing adequate level of optical aberration correction. This will inevitably necessitate a complex optical design. However, with the introduction of a field lens of the proper design, the complexity optical design can be substantially simplified, affording a system that can be economically produced, while maintaining a high level of optical performance.